1. Field of the Invention
This invention relates in general to subsea well drilling, and in particular to an apparatus and method for a tieback of a subsea wellhead housing to a production tree at the surface.
2. Description of the Prior Art
In one type of subsea well drilling, a floating vessel will drill the well, cap the well, and then move from the location. Subsequently, the well will be completed by a tieback to a production platform at the surface. In this method, the subsea well will have an outer wellhead housing at the sea floor located at the upper end of a string of large diameter conductor pipe which extends into the well. An inner wellhead housing lands in the outer wellhead housing. Casing secured to the inner wellhead housing extends to a greater depth in the well than the conductor pipe.
The operator drills through the inner wellhead housing to an even greater depth and installs another string of casing. This string of casing is supported by a casing hanger that lands and seals in the inner wellhead housing. Frequently, the operator will then drill to a final depth and install another string of casing supported by another casing hanger which will land in the inner wellhead housing and seal to the bore of the inner wellhead housing. Then, the floating drilling vessel will cap the well and move from the location.
At a later date, a production platform will be installed at the surface in one type of offshore production. The operator will lower a funnel onto the inner wellhead housing, the funnel being located on the lower end of a string of riser. One type of tieback utilizes an external connector that connects the riser to the inner wellhead housing. Another type utilizes an internal connector to connect the funnel and lower end of the riser to the inner wellhead housing. The internal type is generally less expensive, however it has a bore which is smaller than that of the wellhead housing and therefore restricts access to the casing hangers suspended inside the wellhead housing.
An internal wellhead housing tieback connector is shown in U.S. Pat. No. 4,976,458, issued Dec. 11, 1990, Stanley Hosie et al. It employs a lockdown sub or tieback sleeve that is carried in an upper position and will move to a lower position when engaged by a running tool. In the lower position, the tieback sleeve will engage a tieback profile formed in the bore of the inner wellhead housing to tightly pull the funnel into engagement with the rim of the inner wellhead housing. The inner diameter of the tieback sleeve is less than the bore of the inner wellhead housing. While this is workable, in some circumstances, operators may wish to have full bore access to the inner wellhead housing.